This relates generally to graphics processing and, specifically, to occlusion culling.
Culling means “to remove from flock,” and in graphics, it boils down to removing work that does not alter the final image. This includes, for example, view frustum culling, where objects that are outside the view frustum are not further processed, since they will not affect the final image.
In maximum depth culling, often called hierarchical occlusion culling, the maximum depth, zmax, of a tile is stored and maintained per tile. If the estimated conservative minimum depth of a triangle inside a tile is greater than the tile's zmax, then the triangle is completely occluded inside that tile. In this case, the per-sample depth values do not need to be read from memory, and no further processing is needed within the tile for that triangle. This technique is sometimes called zmax-culling. In addition, one may also store the minimum depth, zmin, of the depths in a tile, and avoid depth reads if a triangle fully covers a tile, and the triangle's estimated conservative maximum depth is smaller than the zmin, in which case the triangle will overwrite all depths in the tile (assuming no alpha/stencil test etc).
The zmax is computed from the per-sample depths in a tile. Ideally, the zmax-value of a tile should be recomputed every time a depth whose value is zmax is overwritten.
A depth culling unit stores the zmax-representation, which may be accessed through a cache, for example. A tile may hold plane equations from the triangles that covers the tile, and for subsequent triangles culling can be done immediately against that representation (either against the evaluated depths from the plane equations, or by computing zmax of the plane equations inside the tile). When the tile can no longer maintain this representation, due to storage requirements or other criteria, the plane equations may be replaced by one or many zmin- and zmax-values for the entire tile, or for subtile regions of the tile. For example, an 8×8 tile could hold two zmin and two zmax for each of the 4×8 subtile regions. Unless a feedback is used, sending computed zmax values back from the depth buffer unit to the hierarchical occlusion culling unit, the zmax-values needs to be conservatively updated and will in general stay the same, unless an entire subtile is fully covered by a triangle.